1. Field of the Invention
This invention relates generally to a laser modulator for communications and more particularly to a staggered quadrature phase shift keyed laser modulator for communications.
2. Discussion of the Background
In the field of communications great promise has been shown by the possibility of using optical carriers in place of electronic communications. Such optical communication systems in particular have the ability to carry a great many conversations over a much smaller cable than in traditional electronic systems. In order for these communication systems to reach their full potential, it has been necessary to develop various light modulation systems so that input electrical digital data may be sent across optical cables. Corresponding electronic systems have used a great many types of coding systems for the transmission of digital data. One such type of coding is the phase shift keyed modulation.
U.S. Pat. No. 4,338,579 to Rhodes shows such a frequency shift offset quadrature quaternary phase shift keyed transmission. This patent shows an electronic communication system which utilizes the sum of two quadrature carriers which are separately modulated and staggered in time by one half of a symbol interval. While this technique has many attributes, it requires the use of a clock system in order to gate the pulses. It also is directed for use in electronic systems rather than the preferable light communication system.
U.S. Pat. No. 3,956,626 to Ross shows a pulse quaternary communication system with a light communication system utilizing a pulse quadrature modulation technique. This system uses a combination of polarization and delays in order to code the light pulses. However, this system does not utilize phase shift keyed modulation.
U.S. Pat. No. 4,006,418 shows a phase shift keying quaternary communication system. Two quadrature signals are summed with one of the signals being delayed by a half bit so that four levels from -135.degree. to +135.degree. are possible. The output is applied to a microwave radiolink. The system also depends on a clocking system to provide for appropriate timing.
Various forms of phase shift keyed modulation systems have been developed for electronic systems. However, the application of these devices to optical communication systems has not been completely successful. Even with the electronic systems, clocks and other timing circuits are necessary in order to make the logic work properly.